United States President Donald Trump has pledged to help ZTE Corp "get back into business, fast" after a US ban crippled the Chinese technology company, offering a job-saving concession to Beijing ahead of high-stakes trade talks this week.

"Too many jobs in China lost. Commerce Department has been instructed to get it done!" Mr Trump wrote on Twitter in the first of two tweets about US trade relations with China.

Shortly after Mr Trump's tweet, a Democratic politician questioned the move to help the Chinese company, given numerous warnings about ZTE's alleged threat to US national security.

ZTE suspended its main operations after the US Commerce Department banned American companies from selling to the firm for seven years as punishment for ZTE breaking an agreement reached after it was caught illegally shipping US goods to Iran.

The Commerce Department, ZTE and the Chinese embassy in Washington could not immediately be reached for comment.

White House spokeswoman Lindsay Walters confirmed that US officials were in contact with Beijing about ZTE.

She said Mr Trump's tweet underscored the importance of "free, fair, balanced and mutually beneficial" relations between the US and China on issues involving the economy, trade and investment.

Washington and Beijing have proposed tens of billions of dollars in tariffs in recent weeks, fanning worries of a full-blown trade war that could hurt global supply chains and dent business investment plans.

In trade talks in Beijing this month, China asked the US to ease crushing sanctions on ZTE, one of the world's largest telecommunications equipment makers, according to people with knowledge of the matter.

In a second tweet on Sunday, Mr Trump said past US trade talks with China posed a hurdle that he predicted the two countries would overcome.

"China and the United States are working well together on trade, but past negotiations have been so one sided in favour of China, for so many years, that it is hard for them to make a deal that benefits both countries," Mr Trump wrote on Twitter.

"But be cool, it will all work out!"

An unprecedented reversal

The US Government launched an investigation into ZTE after Reuters reported in 2012 the company had signed contracts to ship hardware and software worth millions of dollars to Iran from some of the best-known US technology companies.

ZTE pleaded guilty last year to conspiring to violate US sanctions by illegally shipping US goods and technology to Iran and entered into an agreement with the US Government.

The ban is the result of ZTE's failure to comply with that agreement, the Commerce Department said.

Other experts said Mr Trump's policy reversal was unprecedented.

"This is a fascinating development in a highly unusual case that has gone from a sanctions and export control case to a geopolitical one," said Washington lawyer Douglas Jacobson, who represents some of ZTE's suppliers.

Mr Trump's announcement drew sharp criticism from a Democratic lawmaker, who said the move was jeopardising US national security.

"Our intelligence agencies have warned that ZTE technology and phones pose a major cyber security threat," Representative Adam Schiff, a Democrat, said on Twitter.